Field of the Invention
The present invention relates to a device and to a method for pumping a cryogenic fluid.
The invention relates more particularly to a device for pumping a cryogenic fluid, comprising a storage tank for storing a cryogenic fluid containing cryogenic liquid, a cryogenic pump having an inlet head loss (NPSH), and a suction line connecting the tank to the pump.
The invention finds a particularly advantageous application in the field of the pumping of low-density cryogenic fluids containing gases such as hydrogen or helium, and isotopes thereof.
Related Art
Compressing liquid hydrogen makes it possible to reduce the compression costs by comparison with compressing gaseous hydrogen given that it is easier to compress a volume of incompressible liquid than a volume of gas.
Generating this high pressure is extremely expensive in terms of compression energy. In addition, the evaporative losses of liquid hydrogen in a pump may also be high if the pump is not used optimally. Reducing the losses (both friction losses and gas losses) is therefore a key issue in optimizing the costs of obtaining high-pressure hydrogen.
One of the problems with cryogenic pumps in general and with liquid hydrogen pumps in particular is that the fluid that is to be pumped is very low density (70 g/l at 1 bar). It is therefore difficult if not impossible to provide the pump with the suction pressure it requires simply by physically installing the source tank on the pumping installation with a head of pressure (hydrostatic head). The problem is that the suction pressure has to take account of the pump inlet head loss (NPSH=Net Positive Suction Head, that is to say the difference in pressure between the saturation pressure of the gas that is to be pumped and the fluid suction pressure needed for the pump to operate in a pure liquid phase without cavitation).
For example, a liquid hydrogen (LH2) pump at 700 bar has a head loss (NPSH) of around 250 mbar, which corresponds to a 35 m head of liquid hydrogen. It is impossible to run the pump with a source tank installed on the pump with a pressure head of 35 m (and even if this were industrially possible, the head losses in the lines would counterbalance the fact that the tank had been installed with such a pressure head). One solution is therefore to “supercool” the liquid and to suck up this liquid in its supercooled state. Supercooling involves increasing the pressure of a fluid to saturation or reducing its temperature, at constant pressure, without waiting for a new liquid-vapor equilibrium to become established.
Pressurized hydrogen, however, is even less dense than hydrogen at atmospheric pressure. For example, the density of saturated hydrogen at 1 bar absolute is 70 g/l whereas it is 56 g/l at 7 bar absolute. Given that liquid hydrogen pumps are positive-displacing systems, it is therefore beneficial to suck up the hydrogen when it is as dense as possible, and therefore when it is saturated at the lowest possible pressure (as cold as possible), the purpose of this being to optimize the quantities pumped.
The invention described hereinbelow notably makes it possible to use a liquid hydrogen pumping plant continuously from a hydrogen source in liquid/gas equilibrium at a low pressure (of between 1 and 12 bar) and to optimize the operation of such a plant by allowing the pump to operate continuously while at the same time maximizing the density of the pumped hydrogen, and therefore maximizing the pumped output.
In existing solutions, the tank is pressurized using thermosiphon (a heater that establishes atmospheric pressure) or directly using high-pressure hydrogen from cylinders at ambient temperature.
During the running of these known systems, the hydrogen at ambient temperature injected into the roof of the tank gradually heats up the liquid, reducing the available level of supercooling.
This then increases the rated pressure of the tank, with the effect of reducing the pumping time available before the tank reaches its maximum operating pressure.
Document WO2005/085637A1, in the name of the applicant company, notably describes a pumping system comprising pressure control means capable of keeping the pressure in the suction line of the pump at most equal to the saturation pressure of the cryogenic fluid increased by the inlet head loss of the cryogenic pump.
It is one object of the present invention to alleviate all or some of the abovementioned disadvantages of the prior art.